HHV-6 and HHV-7 are newly recognized lymphotropic herpesviruses. HHV-6 is the causative agent of the childhood disease roseola infantum and recent evidence has suggested that it might be involved in infant hepatitis and in complex roseola cases with viral encephalitis. HHV-7 was isolated last year in our laboratory from CD4+ T cells of a healthy individual. Significant efforts this past year have been devoted to the development of epidemiological approaches and tests for these viruses. Examination of sera of healthy adults and children has shown that HHV-6 and HHV-7 are highly prevalent viruses which infect in early childhood. Seroconversion for HHV-7 occurs at a later age than seroconversion for HHV-6. Examination of virus isolates obtained from roseola patients as well as by reactivation of peripheral blood lymphocytes of healthy individuals, revealed that HHV-6 strain are highly conserved viruses, but they fall into two distinct groups of viruses which differ in their growth properties, restriction enzyme patterns, antigenic reactivity and disease association. Only one of these virus groups (prototyped by Z29 strain) appears to be associated with roseola infantum. It is as yet unknown whether viruses of the second virus prototyped by the U1102 strain infect in early childhood or are associated with any disease. In additional studies regarding disease association of HHV-6 and HHV-7 we have found no evidence for involvement of HHV-6/7 in Kawasaki Disease. Moreover, there was no compelling evidence for the involvement of HHV-6 or HHV-7 in chronic fatigue syndrome. Occasional elevated titers were noted but they may have reflected virus reactivation perhaps due to immune impairment in these patients. Studies designed to HHV-6/7 with additional diseases are in progress.